Polyimide matrix electrolyte

ABSTRACT

A polymer matrix electrolyte (PME) includes a polyimide, at least one salt and at least one solvent intermixed. The PME is generally homogeneous as evidenced by its high level of optically clarity. The PME is stable through harsh temperature and pressure conditions. A method of forming a PME includes the steps of dissolving a polyimide in at least one solvent, adding at least one salt to the polyimide and the solvent, wherein said polyimide, salt and solvent become intermixed to form the PME, the PME being substantially optically clear.

CROSS REFERENCE TO RELATED APPLICATION

Not applicable.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

Not applicable.

FIELD OF THE INVENTION

The invention generally relates to electrolytes, and more particularlyto optically clear polyimide based electrolytes which are homogeneousmixtures of polyimide, salt and solvent.

BACKGROUND

Many performance parameters of batteries are associated with theelectrolyte separator selected, and the interaction of the selectedelectrolyte with the cathode and anode materials used. All batteriesrequire electrolytes to provide high ionic conductivity andelectrochemical stability over a broad range of temperatures andpotentials. Ionic conductivity is one of the most important propertiesof electrolytes. High ionic conductivity generally improves batteryperformance. Thus, significant research has focused on developingmethods for increasing the ionic conductivity of electrolytes used inelectrochemical cells.

The electrolyte used in lithium batteries can be a liquid or a polymerbased electrolyte. Lithium batteries including liquid electrolytes havebeen on the market for several years. Lithium ion rechargeable batterieshaving liquid electrolytes are currently mass produced for applicationssuch as notebook computers, camcorders and cellular telephones. However,lithium batteries having liquid electrolyte technology have severalmajor drawbacks. These drawbacks relate to cost and safety and stem fromuse of a liquid electrolyte. The liquid electrolyte generally requirespackaging in rigid hermetically sealed metal “cans” which can reduceenergy density. In addition, for safety reasons, lithium ionrechargeable batteries and lithium-metal primary batteries having liquidelectrolytes are designed to vent automatically when certain abuseconditions exist, such as a substantial increase in internal pressurewhich can be caused by internal or external overheating. If the cell isnot vented under extreme pressure, it can explode because the liquidelectrolyte used in liquid Li cells is extremely flammable.

Lithium batteries having solid polymer electrolytes represent anevolving alternative to lithium batteries having liquid electrolytes.Solid polymer electrodes are generally gel type electrolytes which trapsolvent and salt in pores of the polymer to provide a medium for ionicconduction. Typical polymer electrolytes comprise polyethylene oxide(PEO), polyether based polymers and other polymers which are configuredas gels, such as polyacrylonitrile (PAN), polymethylmethacrylate (PMMA)and polyvinylidine fluoride (PVDF). The polymer electrolyte generallyfunctions as a separator, being interposed between the cathode and anodefilms of the battery.

Because its electrolyte is generally a non-volatile material which doesnot generally under normal operating conditions leak, a lithium batteryhaving a polymer electrolyte is intrinsically safer than a lithiumbattery having a liquid electrolyte. Moreover, polymer electrolyteseliminate the need for venting and package pressure control which aregenerally required for operation of lithium batteries having liquidelectrolytes. Thus, polymer electrolytes make it possible to use a softouter case such as a metal plastic laminate bag, resulting inimprovement in weight and thickness, when compared to liquid electrolytecan-type Li batteries.

Many performance parameters of lithium batteries are associated with theelectrolyte choice, and the interaction of the selected electrolyte withthe cathode and anode materials used. High electrolyte ionicconductivity generally results in improved battery performance. Theionic conductivity of gel polymer electrolytes have been reported to beas high as approximately 10⁻⁴ S/cm at 25° C. However, it is desirablefor the ionic conductivity of the polymer electrolyte to reach evenhigher values for some battery applications. In addition, it would alsobe desirable to enhance the electrochemical stability of the polymerelectrolyte towards anode and cathode materials to improve batteryreliability, as well as storage and cycling characteristics.

While gel polymer electrolytes represent an improvement over liquidelectrolytes in terms of safety and manufacturability, safety issuesremain because gel polymers trap solvent on its pores and under extremeconditions (e.g. heat and/or pressure) can still escape and causeinjury. In addition, gel polymer electrolytes cannot generally operateover a broad temperature range because the gel generally freezes at lowtemperatures and reacts with other battery components or melts atelevated temperatures. Moreover, electrode instability and resultingpoor cycling characteristics, particularly for metallic lithiumcontaining anodes, limits possible applications for such batteriesformed with gel polymer electrolytes.

Alternative polymer materials have been actively investigated to provideimproved characteristics over available polymer choices. For example,U.S. Pat. No. 5,888,672 to Gustafson et al. ('672 patent) discloses apolyimide electrolyte and a battery formed from the same which operatesat room temperature and over a broad range of temperatures. Thepolyimides disclosed are soluble in several solvents and aresubstantially amorphous. When mixed with a lithium salt, the resultingpolyimide based electrolytes provide surprisingly high ionicconductivity. The electrolytes disclosed in '672 are all opticallyopaque which evidences some phase separation of the various componentscomprising the electrolyte. Although the electrolytes disclosed by the'672 patent can be used to form a polymer electrolyte which provides animproved operating temperature range, ease of manufacture, and improvedsafety over conventional gel polymer electrolytes, it would be helpfulif the electrolyte stability and ionic conductivity could be improved.

SUMMARY OF THE INVENTION

A polymer matrix electrolyte (PME) includes a polyimide, at least onelithium salt in a concentration of at least 0.5 moles of lithium permole of imide ring provided by the polyimide, and at least one solvent,all intermixed. The PME is generally homogeneous as evidenced by itshigh level of optical clarity. As used herein, when the PME is referredto as being “substantially optically clear”, it refers to a PME being atleast 90% clear, preferably at least 95%, and most preferably being atleast 99% clear as measured by a standard turbidity measurement,transmitting through a normalized 1 mil film using 540 nm light.

The lithium salt can be selected from the group consisting of LiCl,LiBr, LiI, LiBOB, LiClO₄, LiBF₄, LiAsF₆, LiPF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂and lithium bis(trifluorosulfonyl)imide (LiTFSi). The concentration oflithium salt can be between 0.5 to 2.0 moles Li per mole of imide ringprovided by the polyimide, or 1.2 to 2.0 moles Li per mole of imide ringprovided by the polyimide.

A repeat unit weight per imide ring of the polyimide can be no more than350, no more than 300, or no more than 250. The polyimide can be solubleat 25° C. in at least one solvent selected from the group consisting ofN-methylpyrrolidinone (NMP), dimethylacetamide (DMAc) anddimethylformamide (DMF).

The ionic conductivity of the PME at 25° C. can be at least 1×10⁻⁴ S/cm,and is preferably at least 3×10⁻⁴ S/cm. The PME provides at least oneabsorption between about 1630 and 1690 cm⁻¹, even though neither thesalt or polyimide provide any absorption peaks in this range.

A method of forming a polymer matrix electrolyte (PME) includes thesteps of dissolving a polyimide in at least one solvent, adding at leastone lithium salt in a concentration of at least 0.5 moles of lithium permole of imide ring provided by the polyimide to the polyimide and thesolvent, wherein the polyimide, salt and solvent become intermixed. ThePME is substantially optically clear which evidences the lack of phaseseparation of its respective components.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the present invention and the features andbenefits thereof will be accomplished upon review of the followingdetailed description together with the accompanying drawings, in which:

FIG. 1 (a)–(m) illustrates the repeat unit structure for severalsuitable polyimides, according to an embodiment of the invention.

FIG. 2 is a table illustrating ionic conductivity and the resulting filmcomposition at 20.5° C. for a PME comprising the polyimide shown in FIG.1 (m), LiTFSi salt (2.2×) and the solvent gamma butyrolactone (GBL),according to an embodiment of the invention.

FIG. 3( a)–(d) are plots of the ionic conductivity of a PME comprisingthe polyimide shown in FIG. 1 (m) and the lithium salt LiTFSi (2.2×) asa function of solvent load and temperature for the solvents TMS, PC,GBL, and NMP, respectively.

FIG. 4 is a plot of ionic conductivity of a PME comprising the polyimideshown in FIG. 1 (m) and the lithium salt LiTFSi (2.2×) as a functionsolvent/polyimide ratio at 20 C, where the solvent is GBL.

FIG. 5( a) is FTIR data for an exemplary polyimide without the additionof a Li salt or any solvent.

FIG. 5( b) is FTIR data for an exemplary Li salt.

FIG. 5 (c) is FTIR data for the exemplary polyimide whose FTIR data isshown in of FIG. 5( a) intermixed with and the exemplary Li salt whoseFTIR data is shown in FIG. 5( b) evidencing the emergence of a doubletabsorption peak at between about 1630 cm⁻¹ and 1690 cm⁻¹.

FIG. 6 is a table which includes dianhydrides and aromatic diamines thatcan be used for making polyimides presented herein.

FIG. 7 is a SEM evidencing a defect free PME.

DETAILED DESCRIPTION

The invention describes a substantially optically clear electrolyteseparator matrix which includes a polyimide, at least one salt and atleast one solvent. The solvent is generally a low molecular weight, lowviscosity liquid which swells the polyimide at low concentration and ata sufficiently high concentration allows the polyimide, salt and thesolvent to become homogeneously mixed. As used herein, the substantiallyoptically clear electrolyte matrix is referred to as a “polymer matrixelectrolyte” or PME. The PME can be used for batteries, supercapacitorsand other applications.

Regarding batteries, the PME can be used for both lithium ion andlithium metal containing batteries. Unlike gel polymer electrolytes,once the PME is formed, there is generally no free solvent oridentifiable pores. For example, using a SEM at a 50 Å resolution, nopores in the PME can be identified. Instead, the solvent is integratedwith the polymer and the lithium salt in a homogeneous and substantiallyoptically clear matrix. In addition, unlike conventional gel polymerswhere the polymer only provides mechanical support, the polymer, saltand solvent comprising the PME all participate in ionic conduction.

The PME provides high current carrying capacity, cycling stability andmaintains this performance level across a wide temperature range, suchas at least from −40° C. to 100° C. For example, batteries comprisingthe PME can withstand high temperatures and pressures with small changesin open circuit voltage and capacity, such as the conditions generallyused in the hot lamination processes used in credit card manufacturing,or even a typical injection molding process.

As used herein, the phrase “substantially optically clear” refers to amaterial being at least 90% clear, preferably at least 95%, and mostpreferably being at least 99% clear as measured by a standard turbiditymeasurement, transmitting through a normalized 1 mil film using 540 nmlight. Optical clarity data for PMEs formed according to the inventionunder these measurement conditions are shown in Example 1 whichdemonstrate at least 99% optical transmission through a 1 mil(normalized) PME film. The high level of optical clarity of the PMEevidences the homogeneity of the PME comprising its respectivecomponents (polymer, salt and the solvent) as any significant phaseseparation would substantially reduce the optical clarity of the film.

The interaction between the salt and polymers according to the inventionproduces a generally highly optically clear film. Solvent addition doesnot generally change the optical clarity of the film. As the saltconcentration is increased, a point will be reached where salt begins toform a separate phase. At that point, the PME film will begin losing itsoptical clarity.

The PME is generally based on one or more polyimides, which unlike otherpolymer-based electrolytes, participate in ionic conductivity throughthe presence of imide rings and other polar groups. Other polymer typeswhich include highly polar groups having functional groups that cancomplex with lithium salts and participate in ionic conduction includepolybenzimidazoles and polyamide-imides. Accordingly, these polymertypes may also include species useful in forming a PME.

Polyimides are reaction products of a condensation reaction betweendiamines and dianhydrides to initially form a poly (amic-acid)intermediate. Either or both the diamine and dianhydride reagent can bemixture of diamine or dianhydride species. The poly amic-acidintermediate is then converted to a fully imidized polyimide.

The properties of the resulting polyimide formed depend on the selectionof the particular diamines and dianhydride monomers. Polyimides aregenerally known to provide high thermal stability, chemical resistanceand high glass transition temperatures, such as 200° C. to more than400° C.

The current invention has identified polyimides distinct from thosedisclosed in the '672 patent and has found the addition of solvent tosome of these distinct polyimides can result in the formation of a PME.Some of the polyimides identified herein provide represent newlysynthesized polymers. Unlike the polymers disclosed in '672, thepolymers disclosed herein form a substantially homogeneous matrixmaterial (PME) when combined with an appropriate concentration of saltand the solvent. The homogeneity is evidenced by the high level ofoptical clarity provided. In contrast, the electrolytes disclosed in'672 are non-homogeneous mixture, as evidenced by their opaqueness whichis indicative of phase separation of the respective components.

The Inventors have identified and synthesized improved polyimides foruse in forming PMEs by qualitatively relating certain polymer parametersto ionic conductivity. Imide ring density is believed to explain whypolyimide films, when loaded with lithium salt, show significant ionicconductivity, even in the absence of solvent. Repeat unit weight perimide ring is one measure of imide ring density and is calculated bydividing the molecular weight of the entire repeat unit of therespective polyimides by the number of imide rings within the respectiverepeat units.

Imide rings may provide the equivalent of a high dielectric-constant tomaterials because of the high electron density provided by the rings.Accordingly, it is believed that the interaction between the imide ringsand the lithium ion is a factor in determining the ionic conductivity ofa PME. Thus, improved polyimides for use as PMEs can be generallyselected for further consideration by first calculating the number ofimide rings (and to a lesser degree other highly polar groups, such assulfone, carbonyl and cyanide) per molecular repeat unit in a givenpolyimide. The more imide ring function present per unit weight, thehigher the average dielectric strength equivalence of the polymer.Higher equivalent dielectric strength is believed to generally lead toimproved salt interaction, which can improve the ionic conductivity ofthe PME.

Alternatively, a quantity roughly inverse to imide ring density referredto as repeat unit weight per imide ring can be calculated to alsocompare the relative concentration of imide rings in polyimides. As therepeat weight per imide ring decreases, imide rings become anincreasingly greater contributor to the repeat unit as a whole. As aresult, as the repeat weight per imide ring decreases, the equivalentdielectric constant and ionic conductivity of the polyimide generallyincrease.

Polyimides which provide high helium permeability may generally producehigher ionic conductivity and thus form better PMEs. At 25° C., themeasured He permeability of most polyimides according to the inventionhas been found to be at least 20 barrers. The He permeability can be atleast 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 or 80 barrers, or more.

Helium permeability may be measured in the following suggested manner.The gas permeability (GP) of a gas component through a given film havinga thickness (I) and area (A) can be determined by applying a pressure(p) across the film which is equal to the upstream pressure minus thedownstream pressure of the gas component and measuring the resultingsteady state rate of gas flow (f) permeating through the film at STP.GP=(f l)/(A p)

Preferred units for GP is (cm²)(sec)⁻¹(cmHg), where 1 barrer is definedas the GP multiplied by 10¹⁰.

It is believed that the maximum ionic conductivity of the PME generallyoccurs when the polyimide/salt/solvent combination creates a completelyhomogenous, clear matrix. Any phase separation is expected to reduce theionic conductivity values because phase separation would increase thetortuosity within the PME.

The maximum electrolyte conductivity of the PME at a given temperaturegenerally occurs when the polyimide/salt/solvent matrix has a ratiowithin a specified range. The optimum salt concentration is generally inthe range of 0.5 to 2.0 moles Li per mole of imide ring for a polyimide.Too little salt generally does not provide a sufficient number of ionsto contribute to the charge transfer. Too much salt generally leads tophase instability and possible precipitation of the polyimide andresulting loss of homogeneity which can be verified by a loss in opticalclarity.

The solvent is believed to play the role of a swelling agent for thepolymer to move the backbone chains slightly apart and therefore enablesgreater ionic diffusion coefficients. The solvent also acts as asalvation carrier for the ions. Typically the solvent is chosen to alsobe stable within the limitations of the type of battery chosen. Forexample, for a lithium ion type battery the solvent must be stable up tothe reduction potential of lithium metal. The amount of solvent can beoptimized depending upon the either the conductivity desired or thesoftening point of the matrix at some required temperature. Solvents canbe selected from a group of solvents including gamma butyrolactone(GBL), propylene carbonate, N-methylpyrrolidinone (NMP),tetrahydrothiophen-1,1-dioxide (TMS), polycarbonate (PC) and dimethylformamide (DMF).

FIG. 1 (a)–(m) illustrates the repeat unit structure for severalpolyimides, according to an embodiment of the invention. The repeat unitfor a polymer referred to as Polyimide A is shown in FIG. 1( a). Thispolyimide can be formed by reacting PMDA [89-32-7] with TMMDA[4037-98-7]. This high molecular weight polymer is soluble in NMP for alimited number of hours thus a lithium salt containing electrolyte couldnot be produced.

The repeat unit for the polymer referred herein to as Polyimide B isshown in FIG. 1( b). This-polyimide can be formed by reacting 90 mole %PMDA [89-32-7], 10 mole % 6FDA [1107-00-2] dianhydrides and the diamineTMMDA [4037-98-7]. The polymer formed was soluble indefinitely in NMP at20% by weight.

The repeat unit for the polymer referred herein to as Polyimide C isshown in FIG. 1( c). This polyimide can be formed by reacting 85.7 mole% PMDA [89-32-7], 14.3 mole % 6FDA [1107-00-2] and TMMDA [4037-98-7].The polymer formed was soluble indefinitely in NMP at 20% by weight.

The repeat unit for the polymer referred herein to as Polyimide D isshown in FIG. 1( d). This polyimide can be formed by reacting 80 mole %PMDA [89-32-7], 20 mole % PSDA [2540-99-0] and TMMDA [4037-98-7]. Thepolymer formed was soluble indefinitely in NMP at 20% by weight.

The repeat unit for the polymer referred herein to as Polyimide E isshown in FIG. 1( e). This polyimide can be formed by reacting BPDA[2421-28-5] and 3,6 diaminodurene [3102-87-2]. The polymer formed wassoluble indefinitely in NMP at 20% by weight.

The repeat unit for the polymer referred herein to as Polyimide F isshown in FIG. 1( f). This polyimide can be formed by reacting 6FDA[1107-00-2] and 3,6-diaminodurene [3102-87-2]. The polymer formed wassoluble indefinitely in NMP at 20% by weight and is also soluble inacetone, GBL, DMAc, and DMF.

The repeat unit for the polymer referred herein to as Polyimide G isshown in FIG. 1( g). This polyimide can be formed by reacting PSDA[2540-99-0] and TMMDA [4037-98-7]. The polymer formed was solubleindefinitely in NMP at 20% by weight and is soluble in GBL.

The repeat unit for the polymer referred herein to as Polyimide H isshown in FIG. 1( h). This polyimide can be formed by reacting 6FDA[1107-00-2] and TMMDA [4037-98-7]. The polymer formed was solubleindefinitely in NMP at 20% by weight and is soluble in acetone, GBL,DMAc, and DMF.

The repeat unit for the polymer referred herein to as Polyimide I isshown in FIG. 1( i). This polyimide can be formed by reacting BPDA[2421-28-5] and 4,4′-(9-fluorenylidene)dianiline [15499-84-0]. Thepolymer formed was soluble indefinitely in NMP at 20% by weight.

The repeat unit for the polymer referred herein to as Polyimide J isshown in FIG. 1( j). This polyimide can be formed by reacting PMDA[89-32-7], 33.3 mole % TMPDA [22657-64-3] and 66.7 mole % TMMDA[4037-98-7]. The polymer formed was soluble indefinitely in NMP at 20%by weight.

The repeat unit for the polymer referred herein to as Polyimide K isshown in FIG. 1( k). This polyimide can be formed by reacting PMDA[89-32-7] and DAMs [3102-70-3]. The polymer formed was soluble only fora short time in NMP.

The repeat unit for the polymer referred to herein as Polyimide L isshown in FIG. 1( l). This polyimide can be formed by reacting PMDA[89-32-7] and 4-isopropyl-m-phenylenediamine [14235-45-1]. The polymerformed was soluble indefinitely in NMP at 20% by weight.

The repeat unit for the polymer referred herein to as Polyimide M isshown in FIG. 1( m). This polyimide can be formed by reacting PMDA[89-32-7], 33.3 mole % DAMs [3102-70-2] and 66.7 mole % TMMDA[4037-98-7]. The polymer formed was soluble indefinitely in NMP at 20%by weight.

Polyimide structures other than the exemplary structures shown in FIG. 1can be good candidates for use in forming a PME matrix. Characteristicsthat a good PI should display, such as solubility in NMP (or othersuitable solvent) to at least 20 wt % and a low repeat unit weight perimide ring can be a first criteria. However, the solubility of the PIitself or the solution stability when combined with a high concentrationof a lithium salt applicable for battery applications is not easilypredictable.

Behavior in solution can not generally be ascertained by simply lookingat the polymer repeat unit structure. However, it can be determined fromlooking at the structure whether the polymer will possibly becrystalline, or whether it would have a high or low relative density inthe solid phase. The polyimides are preferably non-crystalline,low-density repeat structures because they would be most likely to bevery soluble. However, changing a single side group from a methyl to anethyl can cause the properties of the PI to be changed from useful to ofno use. A suggested meaningful screening procedure for alternativepolyimides for battery applications is to take a polyimide that is knownto have a good solubility in the solvent itself and then try tointroduce the lithium salt at 0.5 mole Li per imide ring or more and seeif (i) the resulting PI/salt/solvent solution is clear and homogenousfor a few hours and (ii) the film cast from that solution remainssubstantially clear when most of the solvent (e.g. 95%) has beenevaporated.

Polymers other than polyimides which provide a high relative localelectron density (e.g. comparable to electron density adjacent to imiderings in polyimides) and the potential for charge separation due tomultiple bond conjugation may form a polymer/salt complex and displaycharacteristic absorption peaks based on the complex formed, providedthey are soluble. Thus, absorption spectroscopy, such as FTIR can beused to identify these soluble non-polyimide polymers which showevidence of complex formation for use as a PME.

The PME includes at least one lithium salt. High PME ionicconductivities are generally achieved using a high salt content, such asfrom about 0.5 to 2.0 moles of Li per mole of imide ring for polyimidepolymers along with some solvent. However, the concentration of lithiumsalt can be 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7,1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.3, 2.4 or 2.5 moles Li per mole of imidering of the polyimide.

Salt concentrations are also described herein as a ratio of the weightof the salt to weight of the polymer (e.g. polyimide) excluding thesalt. For example, a 1× concentration corresponds to an equal amount ofsalt and polymer, while a 2× salt concentration corresponds to twice thesalt concentration as compared to the polymer concentration. The lithiumsalt can generally be any lithium salt known in the art. Preferably,lithium salts are chosen from LiCl, LiBr, LiI, Li(ClO₄), Li(BF₄),Li(PF₆), Li(AsF₆), Li(CH₃CO₂), Li(CF₃ SO₃), Li(CF₃ SO₂)₂N, Li(CF₃ SO₂)₃,Li(CF_(3 CO) ₂),Li(B(C₆ H₅)₄, Li(SCN), and Li(NO₃), lithiumbis(trifluorosulfonyl)imide (LiTFSi), LiBOB, and LiSCN. Preferably, thesalt is either Li(PF₆) or LiTFSi.

FIG. 2 is a table illustrating the ionic conductivity at 20.5° C. andthe corresponding film composition for a polyimide based PME comprisingthe polyimide shown in FIG. 1( m), LiTFSi salt (2.2×) and the solventGBL, according to an embodiment of the invention. Room temperature ionicconductivity values for PME films were calculated from measuredimpedance using a complex impedance analyzer. Measurements using theimpedance analyzer were made in the frequency range of 1 MHz to 0.1 Hz.A value for the resistance of the film, R, was taken as the interceptwith the real axis of a Nyquist plot, or by an extrapolation of the highfrequency portion of the response curve to the real axis. Measurementswere also made on an LCR meter at a fixed frequency of 10 kHz where theresistance values were read directly. The ionic conductivity wascalculated using the formula:C(Seimens/centimeter)=t/R×10⁻⁴where t is film thickness in microns and R is the measured filmresistance in Ohms.

As shown in FIG. 2, for the PMEs tested, the maximum ionic conductivityat 20.5° C. was about 1×10⁻⁴ S/cm for a 1× salt concentration and 4×10⁻⁴S/cm for a 2.2× salt concentration, where the PME film comprised 37.9%GBL. The lower table portion shown in FIG. 2 shows the minimum solventlevel required to provide a conductivity of 1×10⁻⁴ S/cm for 1.0, 1.4,1.8 and 2.2× salt concentrations.

The range of solvent in FIG. 2 is expressed as the solvent/PI ratio andruns from about 1 to about 2. The lower value is set be an ionicconductivity minimum for a given application, such as 1×10⁻⁴ S/cm. Themaximum conductivity was found to occur between a solvent/PI ratio of1.5 to 2.0 depending upon the salt concentration.

The upper solvent/PI limit near 2 is controlled by the mechanicalproperties of the PME. At a high solvent/PI ratio the PME film becomesvery soft and may not be usable over an extended temperature range as aseparator for a battery. Applied to lithium metal batteries, As apractical matter, the solvent is chosen to be moderately stable againstlithium metal, high boiling (>150 C), capable of dissolving the chosensalt and swelling the PI, and finally it should have a low viscositywhich promotes a high conductivity for the salt/solvent combination byitself, as well as for the PME.

To optimize the respective PI, salt and solvent concentrations, asuggested procedure is described below. In a first step, a PI species isselected which is soluble in an appropriate solvent, such as NMP or GBL.All the polyimides shown in FIG. 1 except the polyimide shown in FIG. 1(k), are soluble in NMP. In the second step, a desired lithium salt isselected and an initial concentration is selected, such as a mass ratiothat gives one lithium ion per imide ring provided by the selected PI.The solvent concentration can then be varied from a solvent/PI ratio ofabout 0.8 to 2.0 while measuring the ionic conductivity for severalsolvent/PI data points in between. This can be repeated at saltconcentrations both higher and lower. There are generally severalcombinations of PI/salt/solvent by mass that will give similar ionicconductivities, such as within a factor of two.

FIG. 3( a)–(d) are plots of the ionic conductivity of a PME comprisingthe polyimide shown in FIG. 1 (m) and the lithium salt LiTFSi (2.2×) asa function of solvent load and temperature for the solvents TMS, PC,GBL, and NMP, respectively. These curves generally do not show a clearconductivity maximum because the salt content was 2.2×, and as a result,the amount of solvent needed to provide the maximum ionic conductivitywas large and generally impractical.

FIG. 4 is a plot of ionic conductivity of a PME comprising the polyimideshown in FIG. 1 (m) the lithium salt (LiTFSi; 2.2×) as a functionsolvent/polyimide ratio at 20° C., where the solvent is GBL. The ionicconductivity is seen to increase for increasing solvent to polyimideratio up to about a GBL/polyimide ratio of about 1.5, where the ionicconductivity levels off at a value of about 7×10⁻⁴ S/cm.

The ionic conductivity of PMEs based on polyimides is believed to berelated to the degree the polyimides associate with alkali salts,particularly Li salts. This association is likely related to theformation of complexes between polar groups (e.g. imide and benzene) ofthe polyimide and the salt. Evidence for this association can be foundby measuring the absorption frequencies displayed by the polymer whenintermixed with the salt and comparing this data to the absorptionfrequencies displayed by the polyimide and the salt by themselves.

For a polyimide, characteristic absorption frequencies are believed tobe related to the imide rings and benzene rings that comprise thepolyimide backbone. These absorption peaks are at about 1778 cm⁻¹ and1721 cm⁻¹ for the imide ring and 730 cm⁻¹ for the benzene ring. CommonLi salts, including LiPF₆, LiBOB, LiI and LiTFSi do not show absorptionat these frequencies, nor any absorption peaks between about 1630 and1690 cm⁻¹. However, when certain polyimides which associate stronglywith Li salts are combined with those salts, the resultingpolyimide/salt film show the emergence of a very strong doublet with afirst peak at about 1672 cm⁻¹ and a second peak at about 1640 cm⁻¹.

The first absorption frequency is present when a significantconcentration of salt is in the polyimide film, such as at least 33 wt.%, but does not change significantly with increasing salt concentration.Thus, this peak can be used to indicate the interaction, provided asufficient salt concentration is present. The magnitude of the secondpeak is nearly proportional to the ratio of the Li ion concentration tothe imide rings and can be used to assess the extent of the interactionbetween the polyimide and the salt.

FIG. 5( a) is FTIR data for an exemplary polyimide, the polyimide shownin FIG. 1( c), without the addition of a Li salt or any solvent.Absorption peaks related to the imide rings are shown at about 1778 cm⁻¹and 1721 cm⁻¹ and at 730 cm⁻¹ for the benzene ring. This polyimide doesnot show any detectable absorption peaks between about 1630 and 1690cm⁻¹.

Common Li salts, including LiPF₆, LiBOB, LiI and LiTFSi do not showabsorption between 1630 and 1690 cm⁻¹. FIG. 5( b) shows an FTIR forLiBOB confirming the absence of infrared absorption between about 1630and 1690 cm⁻¹.

FIG. 5( c) shows the FTIR the polyimide shown in FIG. 1( c) combinedwith LiBOB. The resulting polyimide electrolyte shows a very strongdoublet with a first peak at about 1672 cm⁻¹ and a second peak at 1640cm⁻¹.

Thus, FTIR measurements taken from exemplary PMEs according to theinvention provide evidence that the lithium salt forms a complex withthe imide rings of the polyimide. This is likely one of the main reasonswhy the polyimide/lithium salt combination, in a dry film, has noapparent structure or crystallinity. The lack of crystallinity can beshown by a flat DCS trace. This evidence has also been found of aninteraction between the lithium salt and the polyimide as the PME filmis optically clear whether as a free standing film or as an overcoatedfilm.

The PME can be used in a wide variety of applications. Both primary andrechargeable batteries formed from either lithium ion or lithium metalanodes can be formed using the invention. The PME may also be used forsupercapacitors, and other applications.

Polyimides are generally prepared by reacting one or more diamines andone or more dianhydride monomers in a step-wise condensation reaction.Preferred diamines and dianhydrides are shown in FIG. 6.

The purity of the monomeric reactants has been found to be important.Aromatic diamines are known to be air sensitive. Thus, a purificationprocess generally includes removing a portion of the oxidized diaminematerial. Following purification, aromatic diamines typically becomewhite. However, absolutely pure diamines are difficult to achieve, andusually unnecessary. Recrystallization of the diamine from a mixture ofalcohol and water has been found to be both effective and sufficient.The collected crystals are preferably dried under a vacuum because thewater used for crystallization may not be easily removable at roomtemperature.

Dianhydrides are also preferably purified and can be purified using atleast two methods. A preferred purification method is recrystallizationfrom acetic anhydride. Acetic anhydride closes reactive anhydride rings.However, removal of acetic acid produced is sometimes difficult. Washingwith ether or MTBE can be used to remove the residual acetic acid. Thedianhydride crystals generally require drying in a vacuum oven. Analternate method is to quickly wash the crystals with dry ethanol toremove open anhydride rings and then to rapidly dry the washed powder ina vacuum oven. This method works for purifying BPDA if the incomingpurity is at least approximately 96%.

The condensation reaction is performed to the poly (amic-acid) stage.One mole of diamine is reacted with about 1.005 to 1.01 equivalents ofdianhydride. Excess dianhydride is generally preferably used to ensurethat the dianhydride reagent determines the end group of the reactionproduct, the dianhydride excess ensuring that polymer chains areterminated with anhydride end groups which are known to maximizestability of the polymer product. In addition, such an excesscompensates for traces of water which can react with anhydride groups.

The solution is preferably about 15 to 20 weight % total in NMP solvent.The diamine is usually dissolved first, preferably under nitrogen,although this is not required for all diamines. The dianhydride is thenadded. A large scale reaction preferably adds the dianhydride inportions because there is some heat of reaction and it is desirable tokeep the temperature below 40° C. For example, a small scale reaction of50 grams could add the dianhydride all at once, provided there is goodinitial mixing.

The dianhydride generally dissolves much more slowly than the diamine.For a small-scale reaction, the best and simplest way to perform thereaction is to use a cylindrical jar with a polytetrafluoroethylenesealed cap. This jar can be shaken by hand in the beginning and then puton a rolling mill at slow speed to turn the jar smoothly. The reactionproceeds best at room temperature over a two to twelve hour period. Oncethe initial reaction is complete, as evidenced by a substantiallyconstant solution color and viscosity, the solution is ready for imidering closure.

The chemical reaction to form the polyimide from the poly (amic-acid)can be performed with 1.1 equivalents of acetic anhydride per imide ringand 1 equivalent of pyridine as a ring closure catalyst. Pyridine can beadded and rolling preferably continued until mixed. The acetic anhydridecan cause a portion of the amic-acid polymer to precipitate.

The polymer should be redissolved before heating. One needs to carefullyheat the jar to more than 80° C. but less than 90° C. It is best to usean oven and occasionally remove the jar to shake. Heating is performedfor 60 minutes at full heat or until the color changes are complete. Thejar is put on a rolling mill to cool while turning. Once the solutionreturns to room temperature, a fully imidized polymer dissolved in NMPresults. However, the solution generally also has some residualpyridine, acetic acid, and may include some acetic anhydride. Thesolution is expected to be stable for reasonable periods of time.

A preferred method for collecting the product from a small-scalereaction is to precipitate the polymer into methyl alcohol. This removesthe solvent load and traces of monomers that did not react. It also canbe used, in conjunction with the observed viscosity, to estimate thequality of the polymerization. A good, high molecular weight polyimidewill precipitate cleanly into methyl alcohol with little or nocloudiness. The viscosity should be high (e.g. 8000 to 10,000 cp).

The precipitated polymer is preferably washed two or more times untilthe smell of pyridine is slight. The polymer can then be air dried for afew hours in a hood and then dried in a vacuum oven at 125° C.overnight. The volume of methyl alcohol is preferably about a gallon per100 grams of polymer.

On a larger scale, water can be used to remove solvents or tracematerials. However, water is expected to be a less efficient than methylalcohol for this purpose. A final soak with methyl alcohol is preferablyadded before drying if water is used with large quantities of polymer.

EXAMPLES

The present invention is further illustrated by the following specificexamples. The examples are provided for illustration only and are not tobe construed as limiting the scope or content of the invention in anyway.

Example 1

Optical Clarity of Commercially Available Polyimide/Salt/Solvent asCompared to Exemplary PMEs According to the Invention

The purpose of this example is to demonstrate the homogenous nature ofthe PME formed from polyimides that have strong interactions with alithium salt, as demonstrated by high levels of optical transmission ofvisible light. PME films were generated from polyimides shown in FIGS.1( b), (c) and (m), while a Matrimid 5218p/salt/solvent was generatedfor comparison. The PMEs and Matrimid based film were cast upon a clearpolyethylene terephthalate support film from NMP and dried for threehours in a vacuum oven at 120° C. The residual solvent content wasapproximately 4 wt %. All PME compositions and the Matrimid formulationwere 32% polyimide, 64% LiTFSi salt, and the remainder being the smallamount of solvent. All measurements were performed using 540 nm light atroom temperature.

Film Thickness % Transmission Polyimide Type (mils) Absorbance for a 1mil film Matrimid 5218 0.70 1.903  0.2% FIG. 1(b) Pl 1.68 0.005 99.3% FIG. 1(c) Pl 0.66 0.000 100% FIG. 1(m) Pl 0.87 0.000 100%

As demonstrated from the table above, the Matrimid polyimide does notform a clear homogenous composition when the lithium salt concentrationis 64%. Moreover, the Matrimid polyimide does not form a clearhomogenous composition even at substantially lower salt concentrations,such at a concentration of 0.5 moles of lithium salt per imide ringprovided by the polyimide. Polyimides including a lithium salt in aconcentration of 0.5 moles (or less) of lithium salt per imide ringprovided by the polyimide generally lack sufficient minimum ionicconductivity to be a useful electrolyte separator for an electrochemicalcell.

Although only data for polyimides shown in FIGS. 1( b), (c) and (m) areprovided above, all polyimides shown in FIG. 1 ((a) through (m)) havebeen shown to form homogenous, substantially clear films having opticalproperties as shown above for polyimides (b), (c) and (m). This PMEproperty is desired for maximum film stability and ionic conductivity.

Example 2

Scanning electron microscope (SEM) micrographs were taken to evaluatethe morphology of the PME according to the invention. FIG. 7 shows two(2) SEM micrographs of the PME which each evidence a substantiallydefect free and a porosity free film. The thickness of the PME filmsshown were both about 75 μm.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as described in theclaims.

1. A polymer matrix electrolyte (PME), comprising: a polyimide, at leastone lithium salt in a concentration of at least 0.5 moles of lithium permole of imide ring provided by said polyimide, and at least one solventall intermixed, wherein said polyimide is soluble at 25° C. in saidsolvent and said PME is substantially optically clear.
 2. The PME ofclaim 1, wherein said PME provides an optical clarity of at least 95%through a 1 mil film of said PME at 540 nm.
 3. The PME of claim 1,wherein said electrolyte provides an optical clarity of at least 99%through a 1 mil film of said PME at 540 nm.
 4. The PME of claim 1,wherein said lithium salt is selected from the group consisting of LiCl,LiBr, LiI, LiBOB, LiClO₄, LiBF₄, LiAsF₆, LiPF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂and lithium bis(trifluorosulfonyl)imide (LiTFSi).
 5. The PME of claim 1,wherein a repeat unit weight per imide ring of said polyimide is no morethan
 350. 6. The PME of claim 1, wherein a repeat unit weight per imidering of said polyimide is no more than
 300. 7. The PME of claim 1,wherein a repeat unit weight per imide ring of said polyimide is no morethan
 250. 8. The PME of claim 1, wherein said polyimide is soluble at25° C. in at least one solvent selected from the group consisting ofN-methylpyrrolidinone (NMP), dimethylacetamide (DMAc) anddimethylformamide (DMF).
 9. The PME of claim 1, wherein an ionicconductivity of said PME at 25° C. is at least 1×10⁻⁴ S/cm.
 10. The PMEof claim 1, wherein an ionic conductivity of said PME at 25° C. is atleast 3×10⁻⁴S/cm.
 11. The PME of claim 1, wherein a concentration ofsaid lithium salt is between 0.5 to 2.0 moles Li per mole of imide ringprovided by said polyimide.
 12. The PME of claim 1, wherein aconcentration of said lithium salt is between 1.2 to 2.0 moles Li permole of imide ring provided by said polyimide.
 13. The PME of claim 1,wherein said salt and said polyimide do not provide any absorption peaksbetween 1630 and 1690 cm⁻¹, said PME providing at least one absorptionbetween about 1630 and 1690 cm⁻¹.
 14. A method of forming a polymermatrix electrolyte (PME), comprising the steps of: dissolving apolyimide in at least one solvent, adding at least one lithium salt in aconcentration of at least 0.5 moles of lithium per mole of imide ringprovided by said polyimide to said polyimide and said solvent, whereinsaid polyimide, said salt and said solvent become intermixed, whereinsaid polyimide is soluble at 25° C. in said solvent and said PME issubstantially optically clear.